Use a brown, tan, gray, or black marker, to 'edge' the tiles. Also, consider using peel-n-stick vinyl floor tiles: 12"-squares, accurately cut, heavy, easy to cut, and usually around $0.59 USD/square foot. Get the cheapest available, as you don't care what the vinyl looks like -- that side will be down. Line your paper up carefully, and burnish carefully.

Better yet, cover the print side of the paper with Clear Contact Paper (vinyl), from the kitchen section of your local Wal-Mart store, before you attach them to the floor tiles. It will basically laminate the paper as well as stiffen it. Makes the tiles much more durable, overall.

Look great, by the way. Edge the tiles, and they will look much improved, Bill. Cheers!

Bill, my bad. I thought at first you kept them as separate pieces, adhered to several separate pieces of foam core. After re-reading your article, I see that you made one large mat, on a continuous piece of foam core.

Now that I understand your method, I would suggest blackening the space surrounding the print piece edges, on the foam core, before gluing the print pieces down. Line them up on the foam board, marking the edges. Use a permanent marker to blacken 2mm on either side of the edge, then glue the pieces in place. Any gaps should be covered by the black marker pen ink, hiding any white patches.

You can even do this after laying down the print pages, but you will get some black overlap, which may be noticeable. I've done that, too. Be careful with the marker, as it is quite easy to slip, and put black lines on your mat…

I ran into your same issue when applying printed castle tower and wall sections, to 3mm cardboard, for heavy duty, 3D models. I began blackening the edges, and joint areas before gluing the print pieces in place, and that works quite well. If anything shows through, the viewer only sees black, not white. Cheers!